The invention relates to a low-pressure mercury vapor discharge lamp,
having a discharge vessel enclosing, in a gastight manner, a discharge space provided with a filling of mercury and a rare gas,
at least a part of an inner wall of the discharge vessel having a,
transparent layer comprising an oxide of scandium, yttrium or a rare earth metal.
In mercury vapor discharge lamps, mercury constitutes the primary component for the (efficient) generation of ultraviolet (UV) light. A luminescent layer comprising a luminescent material (for example, a fluorescent powder) may be present on an inner wall of the discharge vessel so as to convert UV to other wavelengths, for example, to UV-B and UV-A for tanning purposes (sun panel lamps) or to visible radiation for general illumination purposes. Such discharge lamps are therefore also referred to as fluorescent lamps. The discharge vessel of low-pressure mercury vapor discharge lamps is usually circular and comprises both elongate and compact embodiments. Generally, the tubular discharge vessel of compact fluorescent lamps comprises a collection of relatively short straight parts having a relatively small diameter, which straight parts are connected together by means of bridge parts or via bent parts. Compact fluorescent lamps are usually provided with an (integrated) lamp cap.
It is known that measures are taken in low-pressure mercury vapor discharge lamps to inhibit blackening of parts of the inner wall of the discharge vessel, which parts are in contact with the discharge which, during operation of the lamp, is present in the discharge space. Such a blackening, which is established by interaction of mercury and glass, is undesirable and does not only give rise to a lower light output but also gives the lamp an unaesthetic appearance, particularly because the blackening occurs irregularly, for example, in the form of dark stains or dots. By using the oxides mentioned in the opening paragraph, blackening and discoloration of the inner wall of the discharge vessel is reduced to a minimum.
A low-pressure mercury vapor discharge lamp of the type described in the opening paragraph is known from U.S. Pat. No. 4,544,997. In the known lamp the oxides are provided as a thin layer on the inner wall of the discharge vessel. The known transparent layers are colorless, hardly absorb UV radiation or visible light and satisfy the requirements of light and radiation transmissivity.
A drawback of the use of the known low-pressure mercury vapor discharge lamp is that the consumption of mercury is still relatively high. As a result, a relatively large amount of mercury is necessary for the known lamp so as to realize a sufficiently long lifetime. In the case of injudicious processing after the end of the lifetime, this is detrimental to the environment.
The low-pressure mercury vapor discharge lamp according to the invention has a transparent layer which further comprises a borate and/or a phosphate of an alkaline earth metal and/or of scandium, yttrium or a further rare earth metal.
Layers comprising both the oxides mentioned in the opening paragraph and said borates and/or phosphates in accordance with the inventive measure, appear to be very well resistant to the effect of the mercury-rare gas atmosphere which, in operation, prevails in the discharge vessel of a low-pressure mercury vapor discharge lamp. It has surprisingly been found that the mercury consumption of low-pressure mercury vapor discharge lamps provided with a transparent layer according to the invention is considerably lower than in transparent layers of the known low-pressure mercury vapor discharge lamps. By way of example, low-pressure mercury vapor discharge lamps provided with a transparent layer according to the invention were compared with known low-pressure mercury vapor discharge lamps provided with a transparent layer comprising an oxide. After several thousand operating hours, an at least substantially twice smaller mercury content was found in transparent layers according to the invention as compared with the known transparent layers. Said effect occurs both in straight parts and in bent parts of (tubular) discharge vessels of low-pressure mercury vapor discharge lamps. Bent lamp parts are used, for example, in hook-shaped low-pressure mercury vapor discharge lamps. The measure according to the invention is notably suitable for (compact) fluorescent lamps having bent lamp parts.
The transparent layers in the low-pressure mercury vapor discharge lamp according to the invention further satisfy the requirements of light and radiation transmissivity and can be easily provided as very thin, closed and homogeneous transparent layers on an inner wall of a discharge vessel of a low-pressure mercury vapor discharge lamp. This is effected, for example, by rinsing the discharge vessel with a solution of a mixture of suitable metal-organic compounds (for example, acetonates or acetates, for example, scandium acetate, yttrium acetate, lanthanum acetate or gadolinium acetate mixed with calcium acetate, strontium acetate or barium acetate) or of boric acid or of phosphoric acid diluted in water, while the desired layer is obtained after drying and sintering.
An additional advantage of a transparent layer according to the invention is that such layers have a relatively high reflectivity in the wavelength range around 254 nm (in the discharge vessel, mercury generates, inter alia, resonance radiation at a wavelength of 254 nm). Given the refractive index of the transparent layer, which is relatively high with respect to the refractive index of the inner wall of the discharge vessel, such a layer thickness is preferably chosen that the reflectivity at said wavelength is maximal. By using such transparent layers, the initial light output of low-pressure mercury vapor discharge lamps is increased.
In a preferred embodiment, the transparent layer comprises a borate and/or a phosphate of calcium, strontium and/or barium. Such a transparent layer has a relatively high coefficient of transmission for visible light. Moreover, low-pressure mercury vapor discharge lamps with a transparent layer comprising calcium borate, strontium borate or barium borate or calcium phosphate, strontium phosphate or barium phosphate have a good maintenance.
In a further preferred embodiment of the low-pressure mercury vapor discharge lamp according to the invention, the transparent layer comprises a borate and/or a phosphate of lanthanum, cerium and/or gadolinium. Such a transparent layer has a relatively high coefficient of transmission for ultraviolet radiation and visible light. It has further been found that a transparent layer comprising lanthanum borate or gadolinium borate or comprising cerium phosphate or gadolinium phosphate has a good adhesion with the inner wall of the discharge vessel. Moreover, the layer can be provided in a relatively simple manner (for example, with lanthanum acetate, cerium acetate or gadolinium acetate mixed with boric acid or diluted phosphoric acid), which has a cost-saving effect, notably in a mass manufacturing process for low-pressure mercury vapor discharge lamps.
An additional advantage of the use in low-pressure mercury vapor discharge lamps of a transparent layer comprising a borate and/or a phosphate of scandium, yttrium, lanthanum, cerium arid/or gadolinium is that such layers have a relatively high reflectivity in the wavelength range around 254 nm. By using such high-refractive transparent layers and by optimizing the layer thickness of such layers, a low-pressure mercury vapor discharge lamp having an increased initial light output is obtained. Such layers may be used to particular advantage in, for example, low-pressure mercury vapor discharge lamps for radiation purposes (referred to as germicide lamps).
The transparent layer in a low-pressure mercury vapor discharge lamp according to the invention preferably comprises an oxide of yttrium and/or gadolinium. Such a transparent layer has a relatively high coefficient of transmission for ultraviolet radiation and visible light. It has further been found that a layer comprising these oxides is little hygroscopic and has a good adhesion with the inner wall of the discharge vessel. Moreover, the layer can be provided in a relatively easy manner (for example, with yttrium acetate or gadolinium acetate), which has a cost-saving effect.
In practical embodiments of the low-pressure mercury vapor discharge lamp, the transparent layer has a thickness of approximately 5 nm to approximately 200 nm. At a layer thickness of more than 200 nm, there is a too large absorption of the radiation generated in the discharge space. At a layer thickness of less than 5 nm, there is interaction between the discharge and the wall of the discharge vessel. A layer thickness of at least substantially 90 nm is particularly suitable. At such a layer thickness, the transparent layer has a relatively high reflectivity in the wavelength range around 254 nm.
According to a further preferred embodiment one side of the transparent layer facing the discharge space is provided with a layer of a luminescent material. An advantage of the use in low-pressure mercury vapor discharge lamps of a transparent layer according to the invention is that the luminescent layer comprising a luminescent material (for example, a fluorescent powder) has a considerably better adhesion with such a transparent layer than with a transparent layer of the known low-pressure mercury vapor discharge lamp.
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.